Quite right, lending out used books to one reader at a time, just like the private lending libraries that used to be in every suburb when I was a kid.

Why would anyone sue you?

One issue that the plaintiffs might bring up is that I would have no way of knowing if the ebooks that were uploaded to the site were actually obtained within the bounds of current copyright law. (Extreme watermarking might take care of this) But I think this would fall under the safe harbor provisions of the DMCA.